The present invention relates to an image forming device provided with a laminating function.
In a conventional image forming device such as a laser printer, there is provided a developing portion, a transfer portion, and a fixing portion. In the developing portion, an electrostatic latent image is formed on a photosensitive drum based on an imaging information, and a toner image corresponding to the electrostatic latent image is formed on the photosensitive drum. In the transfer portion, an image recording medium such as a sheet is interposed between the photosensitive drum and a transfer member imparted with a transfer bias voltage for transferring the toner image onto the sheet. In the fixing portion, the toner image transferred onto the sheet is fixed by heating the sheet. A sheet feed passage is provided passing successively through the transfer portion and the fixing portion.
In an electrophotographic type image forming device such as the laser printer and a facsimile machine those performing high speed printing, printing data such as text data and graphic data are transmitted from an external device such as a host computer or a personal computer. In the image forming device, the inputted printing data are developed into bit image data, and the developed bit image data are stored in a printing image buffer. In the developing portion, a line-base electrostatic latent image is successively formed on the photosensitive drum at every one dot line corresponding to one raster retrieved from the printing image buffer.
In a recent demand, a sheet protection such as a lamination becomes popular in such a manner that a rectangular sheet such as a card or a drive license card printed with an image is sandwiched and hermetically sealed by a pair of transparent lamination sheet members whose size is slightly greater than the rectangular card. For the lamination process, a lamination device is used where the pair of lamination sheet members are pressed at relatively high temperature.
As described above, the laser printer provides the fixing portion where the sheet is nipped between a heat roller and a pressure roller so as to pressedly melt the toner which has been transferred onto the sheet. To this effect, the fixing portion is adapted to generate a heating temperature of about 150.degree. C. Thus, the fixing portion of the laser printer is available for the lamination process in terms of heating and pressing the lamination sheet members.
In this regards, there has been proposed an image forming device capable of performing a lamination process by allowing the lamination sheet members to pass through the sheet feed passage if the image forming process is not conducted.
With this arrangement, pressing force and heat provided by the fixing portion can be effectively utilized.
However, if the pair of lamination sheet members are merely fed past the sheet feed passage, residual toners on the photosensitive drum may be adhered onto the lamination sheet members even if the developing operation is not performed. As a result, surfaces of the lamination sheet members may be darkened by the toners. In other words, the surface of the lamination sheet members is contaminated with the toner. Then if the contaminated lamination sheet members is subjected to heat lamination, the toner is fixed on the lamination sheet members to provide a fog which is a dark, hazy deposit or veil of uniform density over all or parts of a surface of the lamination sheet members. Such disadvantage may be accelerated if the developing portion is operated for a prolonged period.